IF

4 Articles

JFET Stands In For Triode In This Infinite Impedance Detector

February 10, 2023 by 6 Comments

An “Infinite Impedance Detector” might sound a little like something that [Zaphod Beeblebrox] would use to zip around the galaxy. It’s not, of course, but it is an interesting and useful demodulator for AM radio signals, as [Sebastian Westerhold] over at Baltic Labs explains in the brief but well-done video below.

If you’ve ever browsed through schematics of old vacuum tube radios, [Sebastian]’s JFET-based detector circuit might look strangely familiar. That’s because this demodulator is about as close to a direct translation between a vacuum tube circuit and a silicon circuit as possible. In fact, [Sebastian] even used literature from the triode version of this detector to figure out the values for some of the components. The only active component is a BF256B JFET; the rest are a small handful of resistors and caps. Construction is in the ever-popular ugly style.

The test setup is simple — a function generator set to 455 kHz and modulated with a 1,000 Hz sine wave. The detector demodulates the audio signal very cleanly, judging by the oscilloscope traces. Just for fun, [Sebastian] also tried a 10.7 MHz carrier with a 1,500 Hz audio modulation, and that worked fine too. He also tried a variation on the circuit with an IF transformer on the input. That circuit works just about the same as the transformerless version, although it does provide a little gain.

Earth-shattering stuff? Probably not. But it does show the fun you can have with a scrap of PCB and a few components, and seems like it could easily be the kind of project that would take you down the RF rabbit hole. Thanks to [Sebastian] for sharing this one with us.

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A Superheterodyne Receiver With A 74xx Twist

April 19, 2021 by 27 Comments

In a world with software-defined radios and single-chip receivers, a superheterodyne shortwave radio might not exactly score high on the pizzazz scale. After all, people have been mixing, filtering, and demodulating RF signals for more than a century now, and the circuits that do the job best are pretty well characterized. But building the same receiver using none of the traditional superhet trappings? Now that’s something new.

In what [Micha] half-jokingly calls a “74xx-Defined Radio”, easily obtained discrete logic chips, along with some op-amps and a handful of simple components, take the place of the tuned LC circuits and ganged variable capacitors that grace a typical superhet receiver. [Micha] started by building an RF mixer out of a 74HC4051 analog multiplexer, which with the help of a 2N3904 phase splitter forms a switching mixer. The local oscillator relies on the voltage-controlled oscillator (VCO) in a 74HC4046 PLL, a chip that we’ve seen before in [Elliot Williams]’ excellent “Logic Noise” series. The IF filter is a simple op-amp bandpass filter; the demodulator features an op-amp too, set up as an active half-wave rectifier. No coils to wind, no capacitors to tune, no diodes with mysterious properties — and judging by the video below, it works pretty well.

It may not be the most conventional way to tune in the shortwave bands, but we always love the results of projects that are artificially constrained like this one. Hats off to [Micha] for the interesting trip down the design road less travelled.

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Radio Shack Shortwave Goes Digital

May 20, 2020 by 25 Comments

If you spent the 1970s obsessively browsing through the Radio Shack catalog, you probably remember the DX-160 shortwave receiver. You might have even had one. The radio looked suspiciously like the less expensive Eico of the same era, but it had that amazing-looking bandspread dial, instead of the Eico’s uncalibrated single turn knob number 1 to 10. Finding an exact frequency was an artful process of using both knobs, but [Frank] decided to refit his with a digital frequency display.

Even if you don’t have a DX-160, the techniques [Frank]  uses are pretty applicable to old receivers like this. In this case, the radio is a single conversion superhet with a variable frequency oscillator (VFO), so you need only read that frequency and then add or subtract the IF before display. If you can find a place to tap the VFO without perturbing it too much, you should be able to pull the same stunt.

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SDR IF Experiments

April 18, 2018 by 23 Comments

The R820T tuner IC is used in the popular Airspy software defined radio (SDR) as well as many of the inexpensive RTL SDR dongles. [TLeconte] did some experiments on intermediate frequency (IF) configuration of the chip, and you’ll find his results interesting.

Using 5 million samples per second and the device’s real mode, the tests look at a what comes out when the IC reads a noise source. There are two registers that set the IF parameters, but the tests show the effects these registers have in precise terms.

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